(I) Preamble:
In the DME system referred to above, an on-board emitter/interrogator sends out, at every azimuth, pairs of interrogation pulses whose carrier frequency is characteristic of a ground beacon equipped with a responder. The responder receives the interrogation signal, detects it and recognizes the pairs of interrogation pulses, and utilizes them to form other pairs of response pulses on another carrier frequency which can be received, detected and recognized by the receiver of the on-board apparatus. The time lapse t between the leading edges of the interrogation pulses and the response pulses received on board provides the distance information.
The time t increases linearly as a function of the distance D between the interrogator and the responder; t is not zero at the same time as D since the two equipments and in particular the responder introduce certain delays for the transmission and for the processing of the emitted or received signals. The equipment of the responder is adjusted with the aid of delay lines so as to introduce a predetermined delay time t.sub.o ; thus, the distance D is given by the relationship D=(t-t.sub.o)(c/2) where c represents the speed of light.
In the TACAN system the TACAN (abbreviation of the English designation "Tactical Air Navigation") the interrogating aircraft receives from the responder two messages which enable it to determine its distance D and its bearing G with reference to that responder.
The distance-measuring operation in the TACAN system is assured by means rather similar to those which are put into use in the DME system; accordingly, in the following description there will be considered only, to establish the terms and to specify certain orders of magnitude, the actual characteristics of TACAN as they are defined in the standards "MIL" edited by the U.S. Defense Department.
Every aircraft equipped with an on-board apparatus conforming to the TACAN standards can interrogate a beacon and receive from it information on distance and bearing, which could be very inconvenient in a system for military use.